1. Field of the Invention
The present invention relates to a pacing lead having straight wire conductors therein and more particularly, to a combination lead body and straight wire conductors which have high flexibility and high bendability and which are mounted within the lead body of the pacing lead in a manner minimizing stress and fatigue on the wire conductors therein.
2. Description of the Prior Art
Heretofore straight wire conductors which are embedded within an insulating material or which have a tightly fitting sleeve or coating of insulating material around the conductor have been proposed for use as pacing leads. However, these prior leads were subject to a high failure rate and were not dependable due to the failure of such leads due to the stress and fatigue placed on the wire conductor as a result of flexing or bending of the insulated wire conductor within the body and particularly, within the heart, upon contractions of the heart.
Examples of some straight wire conductor pacing lead assemblies are disclosed in the Grausz U.S. Pat. No. 3,817,241, the Berkovits U.S. Pat. No. 3,825,015 and the Sable U.S. Pat. No. 3,949,757.
The Grausz U.S. Pat. No. 3,817,241 discloses a straight wire conductor carried by a tubing and embedded in the wall of the tubing so that it is electrically insulated from the interior walls and exterior walls of the tubing.
The Sable U.S. Pat. No. 3,949 757 discloses a catheter for atrio-ventricular pacing which includes one straight wire conductor embedded in an oval-in-cross-section lead body and an insulated straight wire conductor received in a larger-in-diameter lumen in the lead body.
The Berkovits U.S. Pat. No. 3,825,015 discloses a plurality of straight insulated wire conductors within the hollow interior of a catheter.
Some other medical electronic devices for uses other than long term pacing (30 days or longer) which utilize straight wire conductors previously have been proposed. See for example, the Kline U.S. Pat. No. 4,172,451.
As a result of their high failure rate, straight wire conductors have not been widely used in pacing leads. Instead coiled wire conductors have been used since a coil is able to withstand a greater number of flexes or bends within the human body, such as caused by contractions of the heart, without any failures, cracking or breaking of the wire conductor occuring over the expected life of the pacing lead.
A disadvantage, however, of using a coiled conductor is that the wire conductor must have a sufficient diameter to provide the coiled wire conductor with sufficient strength to withstand flexing or bending thereof and to provide the coiled wire conductor with sufficient conductivity for the conduction of electric current therethrough. Also the lumen defined within the coiled wire conductor must be large enough to receive a stiffening stylet therein.
Furthermore, in a bipolar lead one coiled wire conductor is surrounded by a sheath, sleeve or tube of insulating material and a second outer coiled wire conductor is received around that sheath and then has an outer sheath, sleeve or tube of insulating material therearound. This results in a pacing lead body which has a significant thickness.
It will be appreciated that it is desirable to have a minimal thickness of the lead body in view of the limitations placed on the thickness of the lead body by the inner diameter of the blood vessels through which the lead body will be inserted to reach a heart chamber. Typically the diameter of a lead body is approximately 0.050 inch for a unipolar lead having one coiled conductor and one tip electrode and approximately 0.082 inch for a bipolar lead having two coiled conductors and two electrodes at the distal end of the lead.
It also will be appreciated that the greater the diameter of the lead body, the greater its impedance to the flow of blood through a blood vessel and the more rigid the lead will be, rendering it more prone to breakage.
As will be described in greater detail hereinafter, the pacing lead of the present invention provides a pacing lead body of minimal thickness or diameter and containing one or more straight wire conductors which are constructed and arranged so as not to have the high failure rate of the earlier straight wire conductor pacing leads and which have the advantages of the earlier straight wire pacing leads of a minimal outer diameter or thickness which is less than, or not greater than, the thickness of current pacing leads having coiled conductors therein. This is achieved by providing the lead body with a lumen for receiving each wire conductor that has a greater diameter than the wire conductor received therein so that an uninsulated, relaxed, uncoiled wire conductor can be loosely and slidably received within the lumen of the lead body. Also each wire conductor is made of a material which has high flexibility and bendability. One such type of material is a memory effect alloy, such as the equiatomic nickel-titanium alloy. These alloys have an austenite-martensite transformation. In the high temperature or austenite phase, the material exhibits a standard stress-strain curve and exhibits a unique stress-strain curve for the same alloy in its martensitic state. This unique curve exhibits a low strength yield point which represents the re-orientation of the martensitic plates, but does not introduce any work hardening or other degradations to the mechanical fatigue life. The strain induced is recoverable and as long as the device is stressed at a strain, less than the limit designated by a point on the unique curve, no loss of fatigue life is observed. In this application, an equiatomic alloy of nickel and titanium is used. Additions of other elements such as cobalt or vanadium are used to vary the transition temperature between the austenite and martensite phase.
Heretofore such a lead has not been proposed. However, the use of a titanium-nickel alloy rod in a catheter for giving it a curved shape has been proposed in the Wilson U.S. Pat. No. 3,890,977.